The present invention relates to an optical coherent communication system, and particularly to a technique for controlling the intermediate frequency in an optical heterodyne detection system.
Optical heterodyne communication (Optical coherent communication) systems are advantageous in that a long distance, high capacity transmission is possible because reception sensitivity is much higher compared with a direct detection communication system and because it is suitable for a frequency division multiplexed system.
In an heterodyne detection system, a photo detector receives a composite signal of an input optical signal transmitted from a transmitter and a local oscillation light from a local oscillation light source included in an optical receiver thereof. As a result, a beat signal corresponding to a frequency difference therebetween appears at an output of the photo detector as an electric intermediate frequency (IF) signal. By demodulating this IF signal, a base band signal is obtained. When the relative frequency difference between the signal light and the local oscillation light is not maintained constant, fluctuation of the IF occurs, resulting in an error in the demodulated signal. Therefore in the optical heterodyne detection system, the IF frequency has to be stabilized by controlling the local oscillation light frequency.
For this IF stabilization, an automatic frequency control (AFC) technique is used. In conventional optical heterodyne receivers, for realization of AFC, an IF signal is put into a frequency discriminator with a characteristic zero-crossing at a desired IF value and the output signal from it is fed back as an injection current to a semiconductor laser working as a local oscillation light source.
Additionally, in the optical heterodyne receiver, with the same IF frequency, the frequency of the local oscillation light can be higher in some cases or lower in other cases than the frequency of the signal light. Correspondingly there are real and image bands. The characteristics of IF frequency vs. frequency discriminator output voltage utilized for AFC in the real and image bands are opposite to each other, and stable AFC points appears in both bands. A demodulated signal in the image band has poor quality as well known in the art.
The method for pull-in to a desired stable AFC point in the real band is disclosed in U.S. Pat. No. 5,046,140. This method involves sweeping the frequency of the local oscillation light while making a slight frequency modulation of local oscillation light, detecting the differential rate of the output signal voltage of a frequency discriminator using a lock-in amplifier, or the like, and deciding whether the point in is the real or image band based on the result of a positive or negative value of the differential rate.
This method however takes a long pull-in time since it needs to sweep the frequency of the local oscillation light over the whole band including both real and image bands. This long pull-in time, particularly when receiving a frequency-division multiplexed signal with need a for channel switching, places a severe restriction on operation of the system.